Piezoelectric crystal



June 27, 1950 A. M. ROBINSON 2,512,878

PIEZOELECTRIC CRYSTALS Filed Dec, 28. 1945 jZNVEN 4-1: a:

Patented June 27, 1959 rmzosmc'rarc cars'rsr.

Arthur M. Robinson, Boston, Masa, assignor, by mesne assignments, to The Brash Development grlaliinpany, Cleveland, Ohio, a corporation of Application December 28, 1945, No. 637,779

a clai (or. 171-327) The present invention relates to the protection of the surface of piezoelectric crystals and especially plates and assemblies of plates made from water-soluble piezoelectric crystals so that the surface between the electrodes and between adjacent plates shall remain in a permanently non-conductive condition even in the presence of moisture which would otherwise impair the performance of the crystal. Such .plates have heretofore been generally provided with foil'.or other electrodes on their two major faces for establishing an electrostatic field in the plate, and face-to-face assemblies of such plates with electrodes-secured thereto between their adiacent faces are widely used in microphones, phonograph pickups, etc. Such multiple plate assemblies are described in the reissue patents to Charles B. Sawyer No. 20,213, datedDecem-i ber 22, 1936, and No. 20,680, dated March 29, 1938, the patent to Alfred L. W. Williams, No. 2,106,143, dated January 18, 1938, and the patent to John H. Ream, No. 2,266,333, dated Decem-' ber 16, 1941. The performance of these devices may'be seriously impaired when minute amounts of moisture. exist on the edges of the plates or assemblies; The electrical leakage paths between electrodes are short and wide, conditions generally unfavorable for maintenance of high insulation resistance, and in the case of water soluble salt crystals the leakage problem is unusually severe because any moisture present dissolves some of the salt to form a solution of a much higher electrical conductivity than that of water. In extreme cases, sufiicient crystalline material may be dissolved, both on the edges and under the electrodes to destroy permanently the usefulness of the crystal. Piezo-electric crystals of ance to the transfer of moisture or moisture vapor.

Prior to the present invention, a certain degree of moisture protection has been achieved in commercial practice by .wrapping the crystal in metal foil so as to leave a space between the metal foil and the electrode and then coating with asphalt or similar material. From a production point of view, this method is time-consuming, expensive and requires special skill for the satisfactory application of the metal foil.

Attempts have been made to protect such crystals by a coating of certain synthetic resins, such as'polystyrene, which do not absorb moisture.

this type usually have a low melting point and often may be damaged when subjected to a temperature substantially lower than their melting temperature.

" An illustrative example of this type of crystal is a Rochelle salt crystal having a melting point between 70-arid 0 C. but which may be damaged and rend ed unsuitable as a piez o-electric crystal if, subjected to a temperature asgreat as about 55 C. Consequently, it has not been possible to provide crystals of this character with a coating of a moisture proofing material under conditions such that the crystal is subjected to the melting or fusing temperature of the coating material. For various reasons well known to the art, the film thickness of the protective coating should not exceed 0.005 inch. This necessitates coating the crystal with a material which-possesses far more than ordinary resist- Such coatings having a thickness of 0.005 inch or less'have failed to provide satisfactory protection because, while they do not absorb moisture, they permit the gradual transfer of sum cient moisture to the crystal to cause it electrical failure.

To assist in a further understanding of the invention a piezo-electric crystal embodying my invention is shown in the accompanying drawings, in which:

Fig. 1 shows two plates with applied electrodes;

Fig. '2 shows the plates superimposed on each other and cemented together;

Fig. 3 shows the assembly after the applica-' tion of an outer conductor;

Fig. 4 shows the assembly after being coated,

and

Fig. 5 shows the assembly after the application of an outer protective coating.

Referring to the drawings,- at It and I2, Fig. 1, are shown two crystal plates whose edges are preferably slightly rounded. To these are cemented electrodes Ii and It, a conductive lead being secured to one of them at ll. The plates are then cemented together as illustrated in Fig. 2 and an outer conductive lead 23 (see Fig. 3), is secured to the plate l2.

. .A composition composed of solvent and the coating material is then prepared. When heated to a temperature below that jatwhich the crystal or other article may be damaged, this is a viscous fluid and at low temperature above normal room temperature forms a gel. The crystal is dipped in such composition while the composition is in its viscous fluid condition and the thus coated crystal then 'is allowed to cool to cause the coating indicated at 25 (Fig. 4) to-gel. If the coated crystal is cooled rapidly so as to cause the coating to gel immediately a film of even thickness and the absence of any pendant drop results. The solvent is removed from the gelled coating at a temperature below the gelling phatic and aromatic hydrocarbon solvents, such as toluol, benzol, heptane, V. M. P. naphtha,- etc. which are sufliciently concentrated to gel at a temperature lower than the destructive temperature ofthe crystal and above the operating temperature of the piezo-electrlc crystal. However, coatings of such waxes are somewhat soft and can be deformed easily under pressure,

but I have discovered that it is possible to apply a hard envelope 26, see Fig. of a material, such as shellac, nitrocellulose, ethyl cellulose, etc. over the micro-crystalline wax coating to prevent deformation of the latter.

The particular concentration of the wax in the solution should be such as to provide a finished coating of desired thickness and to form a gel at a temperature between normal room temperature and the destructive temperature of the crystal. The thickness of the coating produced by a solution of given concentration is controlled by the temperature of application. Consequently, the temperature of application should be maintained carefully within about 0.5 C.

Microcrystalline waxes are essentially hydrocarbons of the paraffin series between Call-I and C43H88 and having a melting point between 70 C. and 84 C. These waxes, also known as amorphous petroleum wax, and petrolatum wax, are produced by solvent separation from residual petrolatums and are distinctly different from the paraflin waxes, or other high melting point brittle vegetable and mineral waxes, in that they contain certain natural materials which prevent growth of crystals commonly associated with the paraillns and vegetable waxes. They are prac: ticaily amorphous and best defined as microcrys- -talline in character. Suitable compositions for use in the practice of the invention may include from between about percent to about 33 percent but, preferably, about 30 percent microcrystalline wax and a hydrocarbon solvent. Such compositions form a thick smooth cohesive gel at temperatures between about 30 C. and about 45 C. depending upon the particular microcrystalline wax and hydrocarbon solvent used.

The following examples show the gelling temperature of compositions of different microcrystalline waxes with different hydrocarbon solvents:

Cone. of Gelling rem-- Wax Wax Solvent mm Per cent "0 20 benzol-- 38.0

33% 44. 0 20 toluol 36.0 Petrosene A" 2% o.---.-

-. n 5, 20 heptane 35. 0 30 (lo 41.0 33% fin 44.0 20 31.0 30 sec Product $102---. 2&3 1!) 38.0 33 39.0

. "Petrosene A" and "Product 2310 are microcrystalline waxes manufactured by the Socony- Vacuum Oil Company.

The physical characteristics of Petrosene A Y are as follows:

Melting point, ASTM, "Min" 166 Needle penetration 771E, ASTM 15-25 Color, Lov 5/10 8. U. Visc. 210 F 60/70" Flash, C. 0. 0., F. (minimum) 460 Gravity, API (Approx.) 36

The physical characteristics of "Product 2310" are as follows:

Melting point, ASTM, F -Min 155 Needle penetration 77 F., ASTM----.. 25-35 Color, Lovibond 5/10 s. U. Vise. 210 F a 60/70" Flash, C. O. C., F. (minimum) 460 Gravity, API' (Approx.) 36

The practice of the invention is illustrated further by the following example of coating a Rochelle salt crystal plate:

A microcrystalline wax known as Petrosene A (supplied by Socony Vacuum Oil Co. Inc.) is

. Butanol i I Crystal plates having dimensions of s" 1" .060" prepared in the above manner have withstood continuous immersion'in water at 100 dissolved in hot toluol to form a 30% solution by weight. The hot solution is filtered. This solution forms a thick gel when cooled to about 41 C. For application to the crystal the solution is heated in an agitated constant temperature bath to 51 C. At this temperature the composition is a viscous fluid. After removing the sharp edges from the Rochelle salt crystal plate, it is coated by dipping in the viscous fluid and immediately withdrawing. When the crystal is withdrawn from the viscous fluid the latter gels thereon almost immediately due to the cooling action of the relatively cool crystal and the relatively cool surrounding atmosphere. Consequently, there is practically no flow down the sides of the crystal. This results in the formation of a film of even thickness upon the crystal. The coated crystal is hung up and allowed to dry to evaporate the toluol for from 3 to 6 days depending upon the Finish No. 1

Orange shellac solution in alcohol 4 lbs. per gallon Finish No. 2

Per cent by wt. V sea-nitrocellulose (30% alcohol) 13.4 Dibutyl phthalate 1.5 Toluol p 1.7 Acetone 27.8 Methanol 27.8 27.3

F. for. over 100 days with failing. Asphalted foil wrapped crystals tested in the same manner were found to fail infrom 38'to 95 hours. A crystal plate. was considered to have failed when the electrical resistance between electrodes covering the two faces of the plate had dropped to one megohm. I

The method of the invention may be utilized for forming a moistureproof coating which is essentially a microcrystalline wax upon any ob- Ject which-may be damaged by contact with the microcrystalline wax in its melted condition. The method is especially useful for coating plates and multiplate assemblie of piezo-electric crystals of Rochelle salt but it is useful also for coating whole crystals, and sections and assemblies of other than plate shape, and is useful with other piezo-electric crystals such as ammonium dihydrogen phosphate and lithium sulphate.

I claim:

1. A piezo-electric crystal having a moisture proof coating which is essentially a microcrystalline wax, said coating having a melting temperature which is above. that at which the crystal would be damaged, and being the residue from a solvent solution of said wax.

2. A piezo-electric crystal having a, moisture proof coating which is essentially a microcrystalline wax, said coating having a melting temperature which is above that at which the crystal would be damaged and being the residue from a solvent solution of said wax, said coating being protected by a layer of a harder material.

3. A piezo-electric Rochelle salt crystal having a melting point between 70 and 80 C. and which is likely to be damaged by a temperature about 55 0., said crystal having a moisture proof coating which is essentially a microcrystalline wax having a melting point between 70-84 C. said coating being the residue from a solvent solution of said wax.

4. A piezo-electric Rochelle salt crystal having a moisture proof coating, said coating comprising an inner layer which is essentially a microcrystalline wax and being the residue from a solvent solution of said wax and an outer protective layer of a harder material.

5. A piezo-electric element comprising a crystal plate and electrode means secured to said plate for establishing an electrostatic field therein, said element having a moisture proof coating which is essentially a microcrystalline wax and completely encloses said plate and is the residue from a solvent solution of said wax, said plate having properties which may be damaged it said plate is contacted with the coating material in melted condition.

6. A pieao-electric element comprising a crystal plate and electrode means secured to said plate for establishing an electrostatic field therein, said element having a moisture proof coating, said coating completely enclosing said plate and comprising an inner layer which is essentially a microcrystalline wax and is the residue from a solvent solution of said wax and an outer protective layer of a harder material, said plate having properties which may be damaged if said plate is contacted with said wax in melted condition.

7. A piezo-electric element comprising a plurality of crystal plates having their adjacent faces secured together, and electrode means associated wi faces of said plates for establishing an electrostatic field in each of said plates, said element having a moisture proof coating which completely encloses the assembled plates and which is essentially a microcrystalline wax and is the residue from a solvent solution of said wax, said plates having properties which may be damaged if said plates are contacted with said wax in melted condition.

8. A piezo-electnic element comprising a plurality of crystal plates having their adjacent faces secured together, and electrode means associated with faces of said plates for establishing an electrostatic field in each of said plates, said element having a moisture proof coating which completely encloses the assembled plates and which comprises an inner layer which is essentially a microcrystalline wax and is the residue from a solvent solution of said wax and an outer layer of a harder material, said plates having properties which may be damaged if said plates are contacted with said wax in melted condition.

AR'I'HUR M. RO'BINSON.

REFERENCES CITED The following references are of record in the 

